Processes for producing iii-n single crystals, and iii-n single crystal

ABSTRACT

The present invention relates to a III-N single crystal adhering to a substrate, wherein III denotes at least one element of the third main group of the periodic table of the elements, selected from the group of Al, Ga and In, wherein the III-N single crystal exhibits, within a temperature range of an epitaxial crystal growth, a value (i) of deformation εXX in the range of &lt;0. Additionally or alternatively, the III-N single crystal exhibits at room temperature a value (ii) of deformation εXX in the range of &lt;0.

RELATED APPLICATIONS

The present application is Divisional of currently pending U.S.application Ser. No. 14/386,833, filed on Sep. 22, 2014, which is a U.S.National Phase of PCT Application PCT/EP2013/055891, filed on Mar. 21,2013, which claims the benefit under 35 U.S.C. § 119(a)-(d) of Germanpatent application No. 10 2012 204 553.8 filed Mar. 21, 2012, and Germanpatent application No. 10 2012 204 551.1 filed Mar. 21, 2012, and whichclaims the benefit under 35 U.S.C. § 119(e) of U.S. provisionalapplication No. 61/614,161 filed Mar. 22, 2012, and U.S. provisionalapplication 61/614,190 filed Mar. 22, 2012. The contents and disclosuresof these prior applications are incorporated herein by reference intheir entirety.

FIELD OF THE INVENTION

The present invention relates to processes for producing compositesubstrates (called “template(s)” in the following) and for producingIII-N single crystals. The processes according to the present inventionallow to produce crack-free III-N single crystals which are inparticular suitable for use as wafers. III denotes at least one elementof the main group III of the periodic table of the elements, selectedfrom the group of Al, Ga and In.

BACKGROUND

III-N single crystals are of great technical importance. A multitude ofsemiconductor devices and optoelectronic devices such as powercomponents, high-frequency components, light-emitting diodes and lasersare based on these materials. Epitaxial crystal growth on a startingsubstrate is frequently carried out when producing such devices, or atemplate is initially formed on a starting substrate, onto which III-Nlayers or respectively III-N boules can be subsequently grown by furtherepitaxial growth. III-N substrates or in particular foreign substratescan be used as starting substrates. When using foreign substrates,stresses and cracks within a III-N layer can occur during the growth dueto the differences between the thermal expansion coefficients ofstarting substrate and epitaxial layer. Thicker layers of up to 1 mm canalso be grown with the aid of partially structured interlayers composedof WSiN, TiN or SiO₂, wherein said thicker layers can be subsequentlyseparated as free-standing layers which typically have plastic,concavely bent c lattice planes and surfaces. In particular in case itis dispensed with an intermediate layer in such a process, at or abovethe interface between starting substrate and epitaxial III-N layervertical and horizontal micro-cracks form, which can expand over timeand which can lead to breaking of the GaN layer during or after thecooling process.

From investigations by Hearne et al., Applied Physics Letters 74,356-358 (1999) it is known that during the deposition of GaN on asapphire substrate an intrinsic tensile stress builds up which increaseswith the growth. An in situ stress monitoring showed that the tensilestress produced by the growth cannot be measurably relaxed by annealingor thermal cycling. This means inter alia that a stress obtained at theend of the growth of the GaN layer will have the same value again aftercooling and reheating to the same (growth) temperature. In Hearne et al.also an explanation of the background, relationships and possibilitiesfor observation of extrinsic (namely generated by different thermalexpansion coefficients between sapphire substrate and GaN layer) andintrinsic (namely generated by growth) stress is given.

In this regard Napierala et al. in Journal of Crystal Growth 289,445-449 (2006) describe a process for producing GaN/sapphire templatesonto which crack-free thin GaN layers are grown by being able to controlthe intrinsic stress in the gallium nitride through the setting of thedensity of gallium nitride crystallites in such a way that stresses inthe thin layers can be released by bending. In this process, however,thick layers cannot compensate the pressure during the growth and tendto breaking despite the bending. Richter et al. (E. Richter, U. Zeimer,S. Hagedorn, M. Wagner, F. Brunner, M. Weyers, G. Trankle, Journal ofCrystal Growth 312, [2010] 2537) describe a process for producing GaNcrystals via Hydride Vapor Phase Epitaxy (HVPE) in which GaN layershaving a thickness of 2.6 mm can be grown in a crack-free manner bysetting the partial pressure of gallium chloride, wherein the obtainedGaN layers exhibit a multitude of V-pits on the surface. A crystal grownwith this process has a thickness of 5.8 mm, it however exhibits longercracks. Brunner et al. in Journal of Crystal Growth 298, 202-206 (2007)show the influence of the layer thickness on the curvature of theepitaxial III-N layer. The growth of GaN and AlGaN, optionally withInGaN compliance layer, on GaN-sapphire template is investigated. It wasfound that for GaN and AlGaN with 2.8% and 7.6% of Al mole fraction theconcave curvature increases during the growth, which according toobservation accompanies the generation of a tensile stress (cf. FIG. 3).Furthermore, the concave curvature increases with rising aluminiumcontent, accordingly the tensile stress further increases. In addition,the influence of a Si-doped indium gallium nitride layer on the growthof an AlGaN layer with 7.6% of Al mole fraction on a GaN buffer layer isshown. For this purpose on the one hand an AlGaN layer with 7.6% of Almole fraction is directly grown onto a GaN buffer layer, and on theother hand a Si-doped indium gallium nitride layer as interlayer isgrown onto a GaN buffer layer, wherein subsequently an AlGaN layer with7.6% of Al mole fraction is grown onto the interlayer. It was thus shownthat the deposition of a Si-doped indium gallium nitride layer onto aGaN buffer layer leads to compressive stress in the crystal. During thisprocess the initially concave curvature of the GaN buffer layer istransformed into a slightly convex curvature in the course of atemperature reduction, and this convex curvature increases during thefurther growth by epitaxially growing an In_(0.06)GaN layer within thesame process. During the subsequent deposition of an Al_(0.076)GaN layeronto this In_(0.06)GaN layer a concave curvature is eventually obtained,which is comparatively lower than the resulting curvature withoutIn_(0.06)GaN interlayer.

E. Richter, M. Gründer, B. Schineller, F. Brunner, U. Zeimer, C. Netzel,M. Weyers and G. Trankle (Phys. Status Solidi C 8, No. 5 (2011) 1450)describe a process for producing GaN crystals via HVPE, wherein athickness of up to 6.3 mm can be reached. These crystals exhibit slantedsidewalls and V-pits on the surface. Moreover, the crystal lattice has aconcave curvature of approximately 5.4 m and a dislocation density of6×10⁻⁵ cm².

US 2009/0092815 A1 describes the production of aluminium nitridecrystals having a thickness between 1 and 2 mm as well as aluminiumnitride layers having a thickness of 5 mm. These layers are described ascrack-free and can be used to cut colourless and optically transparentwafers having a usable area of more than 90% for the application in theproduction of devices and components.

The processes in the above-described prior art have in common that aftergrowth and cooling-down III-N crystals are obtained which are subjectedto strong extrinsic and intrinsic stress, whereby cracks or othermaterial defects can develop, which limit the material quality and theprocessability towards III-N substrates.

SUMMARY OF THE INVENTION

The object of the present invention is to provide production processesfor templates and III-N crystals that enable growth of III-N crystalsunder conditions which minimize the inclusion of material defects andimprove the crystal quality as well as the processability.

This object is solved by processes provided hereinbelow. Furthermore,the present invention provides III-N single crystals adhering to aforeign substrate, as provided hereinbelow. Beneficial uses are definedhereinbelow.

According to the present invention it was surprisingly found that III-Nsingle crystals can be grown crack-free and under controllablecompressive stress in case crystal growth is carried out on a substrateat a first crystal growth temperature and subsequently the temperatureapplied before is changed to a second temperature, and if then furthercrystal growth occurs within the range of the second temperature whichis changed compared with the first crystal growth temperature. The wayhow the temperature applied is changed depends on the kind of thesubstrate chosen for the process. If the substrate has a higher thermalexpansion coefficient compared with the III-N single crystal to begrown, then the change to be applied is a reduction of temperature belowthe first temperature applied before. However, if the substrate has alower thermal expansion coefficient compared with the III-N singlecrystal to be grown, then the change to be applied is a raising oftemperature above the first temperature applied before.

Thereby, it is rendered possible that, according to the invention, in atemplate (i.e. a unit having substrate and a relatively thin III-Ncrystal layer, wherein such a template-unit on its part serves asstarting product for the subsequent production of III-N crystalboules/ingots or of III-N devices) the critical parameters curvature andstress of the template, which have been identified to be important, canbe influenced properly and systematically for advantageouscharacteristics of the template and its further use, whereby especiallya later crack formation using the template according to the presentinvention can be efficiently counter-acted. According to alternativetechnical solutions, for settings relevant to the present invention andfavourable for the further processing of the template it has to be madesure that (i) a curvature difference (K_(s)−K_(e)) to be furtherspecified later is held within the range of ≥0 and in particular >0during at least one growth phase during the template production, or (ii)the produced template in the status at growth temperature is not bowedor essentially not bowed or is negatively (convexly) bowed. According tothe present invention templates can be produced which under epitaxialcrystal growth conditions exhibit no or almost no curvature or anegative curvature and thus only a slight intrinsic stress which provedto be advantageous as starting situation for the further processing.

Accordingly, the process according to the present invention and moresignificantly by observation of the preferred features of the processaccording to the present invention allow an advantageous setting of thestrain in the III-N crystal layer of the template with an ε_(XX) valueat room temperature in an advantageous range of ε_(XX)<0, especially ofparticularly suitable negative ε_(XX) values, which has a veryfavourable effect on the further processing of the template according tothe present invention and which thus constitutes an alternative relevantproduct feature of the template according to the present invention.

Conventional processes so far have shown a completely differentbehaviour. In conventional processes, crystal growth is also carried outat a certain, desired temperature. This temperature correlates with atemperature deemed suitable for the respective III-N material. Even ifit may happened that the temperature is reduced in a growth process inorder to grow another III-N material, nevertheless, at this specific,new temperature at the beginning or during growth is not furtherreduced, but is kept constant. However, in such a conventional case, agiven curvature of the growth surface, irrespective whether convex orconcave, typically continuously increases during the growth.Surprisingly, the process according to the present invention can bearranged such that during a particular growth phase of the III-Nmaterial layer of the template a given curvature decreases despite thefurther growth of the given III-N material layer. Furthermore, as aresult of a continuously increasing curvature in conventional processesa corresponding increasing intrinsic—typically tensile—stress within thecrystal is built up, which optionally already during the further growth,at the latest during the cooling-down from the epitaxial growthtemperature, can lead to micro-cracks and even to breaking. By contrast,in the process according to the present invention an intrinsic—typicallycompressive—stress can be kept low, such that during the continuedgrowth and even during the cooling cracks can be avoided, that is,crack-free growth of III-N crystals can be attained.

As an alternative new possibility, optionally to be applied incombination, to favourably influence and to set under appropriateconditions, according to the invention, the above-described favourablecurvature difference (K_(s)−K_(e)) and/or non-curvature, the essentiallynon-curvature or the negative (convex) curvature is based on theprocedural measure that during the growth, or in an intermittentintermediate stage at the beginning or between the beginning andcontinuation of the growth, of a crystalline III-N material on thesubstrate a mask material is deposited as interlayer on the substrate,which optionally exhibits a III-N nucleation layer, or in the actualcrystalline III-N material at a specific maximum distance from thesubstrate, or respectively from the optionally provided III-N nucleationlayer, and subsequently the growth of the crystalline III-N material iscarried out or continued. The interlayer of the mask material, which ispreferably formed as a single thin, generally very thin layer and theconstitution of which will be further described below, is deposited at amaximum distance from the substrate, or respectively from the optionallyprovided III-N nucleation layer, of 300 nm, preferably at a distance ofbelow 300 nm, more preferably below 250 nm, even more below 100 nm, inparticular up to a maximum of 50 nm.

In this alternative method of the designated setting of the curvaturebehaviour it is paid attention to that mask material as interlayer isdeposited at least in part directly on the substrate or on theoptionally present III-N nucleation layer (i.e. immediately adjacent),or in the crystalline material of the template at a suitable distance tothe main surface of the substrate or the optionally present III-Nnucleation layer (i.e. at the location where the contact with substrateor respectively III-N nucleation layer occurs). This applies also in acase when optionally surface structurings are provided on the substrateand thus the prescribed contact occurs only partly; such surfacestructurings namely relate merely to conventional patternings performedex situ, such as for example the opening of windows, the formation ofstripes or dots and other mask structures, for example by(photo)lithography, thus conventional cases in which the desirablecurvature behaviour cannot be set as according to the invention.Furthermore, the dimensions are different: surface maskings andpatterning performed ex situ typically exhibit a thickness dimension inthe am range, whereas the in situ provided mask material interlayer,which according to the invention is applied optionally orsupplementarily, typically exhibits a thickness dimension in the sub-μmrange.

In a III-N crystal according to the invention it can be avoided thatcracks develop, which limit the material quality and/or theprocessability to III-N substrates. “Crack-free III-N crystal” accordingto the present invention denotes that it exhibits no crack on an area of15 cm² (with 2 inch; 3 mm margin exclusion) at inspection ofrespectively 30 mm² image sections with an optical microscope.

According to the present invention furthermore the microscopic propertyof the deformation (strain) ε_(XX) of the lattice constant a can beinfluenced. In mechanics the deformation ε is generally also referred toas strain tensor, wherein ε_(XX) denotes its first component.

For crystal lattices the strain ε_(XX) is defined as follows:

$ɛ_{XX} = \frac{{{lattice}\mspace{14mu} {constant}\mspace{14mu} a} - {{lattice}\mspace{14mu} {constant}\mspace{14mu} a_{0}}}{{lattice}\mspace{14mu} {constant}\mspace{14mu} a_{0}}$

wherein a is the actual lattice constant in the crystal and a₀ presentsthe theoretical ideal lattice constant, wherein for a₀ typically aliterature value of 3.1892±0.00004 Å can be assumed (according to V.Darakchieva, B. Monemar, A. Usui, M. Saenger, M. Schubert, Journal ofCrystal Growth 310 (2008) 959-965).

Accordingly, the actually present crystal lattice constants can beinfluenced by epitaxial growth of crystal layers under extrinsic stress.For example, a compressive stress can be transferred or imposed to thegrowing crystal by extrinsic stress, whereby, compared to growth withoutstress, lattice constants within the growth plane are contracted.Thereby intrinsic stress is built up within the crystal in a controlledand purposive manner, wherein said stress favourably influences theabove-mentioned properties of deformation and stress at continued orsubsequent crystal growth.

Such templates are excellently suited as starting products for growingfurther epitaxial layers of the III-N system, in particular forproducing thick III-N layers and III-N boules (bulk crystals). Accordingto the invention, it is preferred that III-N crystals of templatesaccording to the present invention have a ε_(XX) value in the range of<0.

Without limiting the invention, in the following a compilation of itemsis given, which describe aspects, further embodiments and particularfeatures of the present invention:

1. A process for producing a template comprising a substrate and atleast one III-N crystal layer, wherein III denotes at least one elementof the third main group of the periodic table of the elements, selectedfrom Al, Ga and In, the process comprising the following steps:

-   a) providing a substrate,-   b) carrying out of growth of a crystalline III-N material on the    substrate at a first crystal growth temperature,-   c) changing the temperature to a second temperature which is changed    compared with the first crystal growth temperature, at which however    crystal growth can occur,-   d) continuing crystal growth for forming of III-N crystal within a    range changed compared with the first growth temperature, without    adding In in this step d),-   with the proviso that the second temperature in step c) is lower    than the first temperature and in step d) the crystal growth is    continued below the first growth temperature, if the substrate used    has a higher thermal expansion coefficient than the III-N crystal to    be grown until step d),-   or that the second temperature in step c) is higher than the first    temperature and in step d) the crystal growth is continued above the    first growth temperature, if the substrate used has a lower thermal    expansion coefficient than the III-N crystal to be grown until step    d).

2. A process for producing a III-N single crystal, wherein III denotesat least one element of the third main group of the periodic table ofthe elements, selected from the group consisting of Al, Ga and In, theprocess comprising the following steps:

-   a) providing a substrate,-   b) carrying out of growth of a crystalline III-N material on the    substrate at a first crystal growth temperature,-   c) changing the temperature to a second temperature which is changed    compared with the first crystal growth temperature, at which however    crystal growth can occur,-   d) continuing crystal growth for forming of III-N crystal within a    range changed compared with the first crystal growth temperature,    without adding In in this step d),-   with the proviso that the second temperature in step c) is lower    than the first temperature and in step d) the crystal growth is    continued below the first growth temperature, if the substrate used    has a higher thermal expansion coefficient than the III-N crystal to    be grown until step d),-   or that the second temperature in step c) is higher than the first    temperature and in step d) the crystal growth is continued above the    first growth temperature, if the substrate used has a lower thermal    expansion coefficient than the III-N crystal to be grown until step    d),-   e) additional epitaxial crystal growth for forming of III-N crystal    at a crystal growth temperature which can be selected independently    from said first and second temperatures, wherein optionally, in this    step e) In may be added, and-   f) optionally separating of formed III-N single crystal and    substrate.

3. The process according to item 1 or 2, characterized in that in thesteps b) to d), MOVPE is used as growth method.

4. The process according to any one of the preceding items,characterized in that in step c), the change, i.e. lowering or raisingof the temperature, sets a temperature difference ΔT (T₁−T₂) within areactor, which value is at least at 10° C., in particular in the rangeof 10−100° C., preferably at least at 20° C., more preferably within therange of 20-50° C., even more preferably within the range of 25−40° C.and in particular at 30° C.

5. The process according to any one of the preceding items,characterized in that the first growth temperature in the reactor iswithin the range of 1000−1100° C., preferably at 1020-1080° C., morepreferably at about 1040° C.

6. The process according to any one of the preceding items,characterized in that in step b), III-N crystallites grow vertically andlaterally.

7. The process according to any one of the preceding items,characterized in that in step c), at a point of time when crystallitecoalescence starts, the temperature is changed and that subsequent tosaid change in temperature, an epitaxial crystal growth proceeds overthe coalescing III-N crystallite, as the case may be set within therange below or above the first growth temperature.

8. The process according to any one of the preceding items,characterized in that in step c), the temperature is lowered to a secondtemperature within the reactor lying within the range of 950-1050° C.,preferably at 990-1030° C., more preferably at about 1010° C.

9. The process according to any one of the preceding items,characterized in that in step d), the growth temperature is below thefirst growth temperature within the range of 950-1075° C., preferably of975-1050° C., more preferably of 990 to 1030° C.

10. The process according to any one of the preceding items,characterized in that after the lowering of temperature, the growthtemperature for the further growth in step d) is substantially keptconstant.

11. The process according to any one of the preceding items,characterized in that during the growth phase in step d), i.e. duringformation of a III-N material layer, at growth temperature a givencurvature of the growth surface decreases continually or intermittently.

12. The process according to any one of the preceding items,characterized in that in step b), a concave curvature of the growthsurface is caused and that in step c), by changing the temperature ofthe growth surface, the concave curvature becomes smaller compared withthe curvature before the changing, or the curvature is eliminated.

13. The process according to any one of the preceding items,characterized in that the curvature difference (K_(s)−K_(e)) of thetemplate has a positive algebraic sign at the beginning (curvature valueK_(s)) and at the end (curvature value K_(e)) of step d).

14. The process according to any one of the preceding items,characterized in that the curvature difference (K_(s)−K_(e)) is at least5 km⁻¹, more preferably at least 10 km⁻¹.

15. The process according to any one of the preceding items,characterized in that the curvature difference (K_(s)−K_(e)) is at most50 km⁻¹, preferably at most 20 km⁻¹.

16. The process according to any one of the preceding items,characterized in that after completion of step d), the substrate is notor essentially not curved or is negatively curved, preferably thecurvature (K_(e)) is within the range of at most 30 km⁻¹.

17. The process according to any one of the preceding items,characterized in that after completion of step d) the substrate isessentially not curved, wherein the curvature (K_(e)) preferably lies inthe range of ±30 km⁻¹.

18. The process according to any one of the preceding items,characterized in that the steps b) to d) are carried out in the growthof only a single (i.e. one and the same) III-N layer of the substrate.

19. The process according to any one of the preceding items,characterized in that after completion of step d), a III-N layer havinga thickness within the range of up to 25 μm, preferably 0.1-10 μm, morepreferably 2-5 μm is deposited onto the substrate.

20. A process for producing a III-N single crystal, wherein III denotesat least one element of the third main group of the periodic table ofthe elements, selected from Al, Ga and In, the process comprising thefollowing steps:

-   aa) providing a template comprising a foreign substrate and at least    one III-N crystal layer, wherein within a temperature range of an    epitaxial crystal growth, the template is not or essentially not    curved or is negatively curved,-   bb) carrying out epitaxial crystal growth of a III-N crystal at a    crystal growth temperature at which the template is not or    essentially not curved or is negatively curved,-   wherein it is preferred that in steps aa) and bb), no In is added,    and-   cc) optionally additional epitaxial crystal growth for forming of a    III-N crystal at a crystal growth temperature which can be set    independently from said crystal growth temperature of step bb),    wherein optionally, in this step cc) indium may be added, and-   dd) furthermore, optionally separating III-N single crystal and    foreign substrate.

21. The process according to item 20, characterized in that aftercompletion of step bb) the proviso of “essentially not curved” or“negatively curved” means that the curvature (K_(e)) is within the rangeof at most 30 km⁻¹.

22. The process according to item 21, characterized in that the templateis essentially not curved, wherein the curvature (K_(e)) preferably liesin the range of ±30 km⁻¹.

23. The process according to any one of the preceding items,characterized in that the substrate used in step a) or respectively aa)is a foreign substrate having a higher thermal expansion coefficientthan the III-N crystal to be grown, selected from the group consistingof LiAlO₂ and sapphire, preferably sapphire.

24. The process according to any one of the preceding items,characterized in that the substrate used in step a) or respectively aa)is a foreign substrate having a lower thermal expansion coefficient thanthe III-N crystal to be grown, selected from the group consisting of SiCand Si.

25. The process according to any one of items 20 to 24, characterized inthat in step aa) the at least one III-N crystal layer contains noindium, and/or that in step bb) during the epitaxial crystal growth ofIII-N crystal no indium is added.

26. The process according to any one of items 20 to 25, characterized inthat the template provided in step aa) is produced by a processaccording to any one of items 1 to 19.

27. The process according to any one of items 20 to 26, characterized inthat the template provided in step aa) is produced in that during thegrowth, or in an intermittent intermediate stage at the beginning orbetween the beginning and continuation of the growth, of a crystallineIII-N material for said III-N crystal layer on the substrate a maskmaterial is deposited as interlayer on the substrate, which optionallyexhibits a III-N nucleation layer, or in the crystalline III-N materialitself at a maximum distance from the substrate, or respectively fromthe optionally provided III-N nucleation layer, of 300 nm, preferably ata distance of below 100 nm, more preferably of up to a maximum of 50 nm,and subsequently the growth of the crystalline III-N material for saidIII-N crystal layer is carried out or continued.

28. The process according to item 27, characterized in that the maskmaterial is a material which is different from the substrate materialand III-N material, on which the III-N growth is inhibited, disturbed orprevented, wherein the mask material is preferably selected from thegroup consisting of Si_(x)N_(y) (wherein x and y respectivelyindependently from each other denote positive numbers which lead tostoichiometric or nonstoichiometric SiN compounds; in particular Si₃N₄),TiN, Al_(x)O_(y) (wherein x and y respectively independently from eachother denote positive numbers which lead to stoichiometric ornonstoichiometric AlO compounds; in particular Al₂O₃), Si_(x)O_(y)(wherein x and y respectively independently from each other denotepositive numbers which lead to stoichiometric or nonstoichiometric SiOcompounds; in particular SiO₂), WSi, and WSiN.

29. The process according to item 27 or 28, characterized in that theinterlayer is formed as a single layer, and/or that the thickness of theinterlayer lies in the nanometer or sub-nanometer range, for examplebelow 5 nm, more preferably below 1 nm, in particular down to below onemonolayer (i.e. 0.2 to 0.3 nm or less).

30. The process according to any one of items 20 to 29, characterized inthat the curvature behaviour of the provided template designated in stepaa) is set in the growth of only a single III-N layer of the template.

31. The process according to any one of the preceding items,characterized in that the starting substrate in step a) or respectivelyaa) has a polished surface.

32. The process according to any one of the preceding items,characterized in that the starting substrate in step a) or respectivelyaa) exhibits a surface structured by lithography or wet chemical etchingor dry chemical etching (e.g. ICP).

33. The process for preparing a III-N single crystal according to anyone of the preceding items, characterized in that the crystal growth atleast in step e) of item 2 or in optional step cc) of item 20—optionallyin all crystal growth steps—is carried out via HVPE.

34. The process for preparing a III-N single crystal according to anyone of items 2 to 33, characterized in that after completion of theepitaxial growth, III-N single crystals are grown having layerthicknesses of at least 0.5 mm, preferably of at least 1 mm, morepreferably of at least 5 mm, in particular of at least 7 mm and mostpreferably of at least 1 cm.

35. The process for preparing a III-N single crystal according to anyone of the preceding items, characterized in that at least one andoptionally more GaN-, AlN-, AlGaN-, InN-, InGaN-, AlInN- orAllInGaN-layer(s) is/(are) deposited for preparing accordingly thickerIII-N layers or III-N single crystals.

36. The process for preparing a III-N single crystal according to anyone of the preceding items, characterized in that the III-N crystallayer on the substrate as well as the thereon epitaxially grown III-Ncrystal are composed of the same III-N material.

37. The process for preparing a III-N single crystal according to anyone of the preceding items, characterized in that at the III-N crystallayer on the substrate as well as on the thereon epitaxially grown III-Ncrystal, no exchange was carried out for the III-component.

38. The process for preparing a III-N single crystal according to anyone of the preceding items, characterized in that the III-N crystallayer on the substrate as well as the thereon epitaxially grown III-Ncrystal respectively form a binary system.

39. The process for preparing a III-N single crystal according to items2 to 38, characterized in that the optional separating III-N singlecrystal and substrate takes place by self-separation, preferably duringcooling from a crystal growth temperature.

40. The process for preparing a III-N single crystal according to anyone of items 2 to 38, characterized in that the optional separatingIII-N single crystal and substrate takes place by means of grinding-off,sawing-off or a lift-off process.

41. The process for preparing a III-N single crystal according to anyone of the preceding items, characterized in that the substrate and theat least one III-N crystal layer is formed such that the III-N crystallayer surface has a convex curvature at room temperature.

42. The process for preparing a III-N single crystal according to anyone of the preceding items, characterized in that within the temperaturerange of an epitaxial crystal growth, the III-N single crystal has avalue of ε_(XX)≤0, preferably a value of ε_(XX)<0.

43. The process for preparing a III-N single crystal according to anyone of the preceding items, characterized in that the III-N singlecrystal has a radius within the range of −4 to −6 m at room temperature.

44. The process for preparing a III-N single crystal according to anyone of the preceding items, characterized in that the III-N singlecrystal has a compressive stress of σ_(xx)<−0.70 GPa at roomtemperature.

45. The process for preparing a III-N single crystal according to anyone of the preceding items, characterized in that when sapphire of athickness of 430 m is used as a substrate and GaN of a thickness of 3.5m is used as III-N crystal layer, the III-N single crystal has acurvature of K_(T)<−170 km⁻¹ at room temperature, preferably within therange of −170 km⁻¹>K_(T)>−250 km⁻¹, wherein when using or setting otherlayer thicknesses, the curvature value lies depending on the respectivelayer thicknesses analogous to the Stoney equation in the followingrange:

K _(T)(d _(GaN) ;d _(sapphire))=K _(T)(3.5 μm;430 μm)×(430 μm/d_(sapphire))²×(d _(GaN)/3.5 μm).

46. The process for preparing a III-N single crystal according to anyone of the preceding items, characterized in that the III-N singlecrystal has a value ε_(XX)≤−0.002 at room temperature, preferably avalue ε_(XX) within the range of −0.002 to −0.004.

47. A process for producing III-N crystal wafers, wherein III denotes atleast one element of the third main group of the periodic table of theelements, selected from Al, Ga and In, the process comprising thefollowing steps:

-   a) carrying out a process according to items 2 to 46 for forming a    III-N single crystal, and-   b) separating the single crystals for forming wafers, optionally a    plurality of wafers.

48. A III-N single crystal adhering to a foreign substrate, wherein IIIdenotes at least one element of the third main group of the periodictable of the elements, selected from Al, Ga and In, characterized inthat the III-N single crystal is defined by one or both of the followingvalues (i)/(ii) of the deformation ε_(XX):

(i) at room temperature the ε_(XX)-value lies in the range of <0,preferably the ε_(XX)-value lies in the range ε_(XX)≤−0.002;(ii) within a temperature range in which an epitaxial crystal growth ofany material and in particular III-N material would occur, theε_(XX)-value is ≤0, preferably ε_(XX) is <0.

49. The III-N single crystal according to item 48, characterized in thatε_(XX) at room temperature lies within the range of −0.002 to −0.004.

50. The III-N single crystal according to item 48 or 49 in the form of atemplate having a layer thickness of the III-N single crystal within therange of up to at most 25 μm, preferably of 0.1 to 10 μm, morepreferably 2 to 5 μm.

51. The III-N single crystal according to any one of the precedingitems, characterized in that the III-N single crystal has a compressivestress of σ_(xx)<−0.70 GPa at room temperature.

52. The III-N single crystal according to any one of the precedingitems, characterized in that the foreign substrate is selected from thegroup consisting of SiC, Si, LiAlO₂ and sapphire, preferably itcomprises sapphire, and more preferably it consists of sapphire.

53. The III-N single crystal according to any one of the preceding itemshaving sapphire as foreign substrate and GaN as III-N single crystal,characterized in that when the sapphire has a thickness of 430 μm andthe GaN-layer has a thickness of 3.5 μm, there is a curvature K_(T)<−170km⁻¹ at room temperature, preferably within the range of −170km⁻¹>K_(T)>−250 km⁻¹, wherein when there are other layer thicknesses ofthe sapphire and/or the GaN-layer, the curvature value lies depending onthe respective layer thicknesses analogous to the Stoney equation in thefollowing range:

K _(T)(d _(GaN) ;d _(sapphire))=K _(T)(3.5 μm;430 μm)×(430 μm/d_(sapphire))²×(d _(GaN)/3.5 μm).

54. The III-N single crystal according to any one of items 46 to 51,characterized in that III denotes Ga and the crystal in growth directionexhibits a lattice constant within the range of a<0.318926 nm, forexample in the range of 0.31829 nm<a<0.318926 nm.

55. The III-N single crystal according to any one of the precedingitems, characterized in that the foreign substrate is removed.

56. The III-N single crystal according to any one of items 48 to 54,characterized in that the crystal is prepared according to the processaccording to items 2 to 46.

57. Use of a III-N single crystal or template according to any one ofitems 48 to 56 for preparing of thicker III-N layers or III-N crystalboules or bulk crystals, which are thereafter optionally separated intoindividual III-N wafers.

58. Use of a III-N single crystal or template according to any one ofitems 48 to 56 for preparing semiconductor elements or electronic andoptoelectronic devices.

59. The use according to item 58 for preparing power components,high-frequency components, light-emitting diodes and lasers.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 schematically shows different stages of the growth process forforming a III-N template according to one embodiment of the presentinvention;

FIG. 2 shows the change of the curvature of the growth surface dependingon process step (thereby, the numbers 1 to 6 correspond to steps (i) to(vi) of the FIG. 1) and respectively applied temperature by way of theexample of the deposition of GaN onto sapphire according to a possibleembodiment;

FIG. 3 illustrates temporal temperature-, reflection- andcurvature-profiles during exemplary growth of GaN on sapphire accordingto one embodiment of the present invention;

FIG. 4 illustrates temporal temperature-, reflection- andcurvature-profiles during exemplary growth of GaN on Si or SiC accordingto an alternative embodiment of the present invention; and

FIG. 5 illustrates temporal temperature-, reflection- andcurvature-profiles during conventional growth of GaN on sapphire.

FIGS. 6A and 6B schematically show further embodiments with whichalternatively templates can be provided whose stress can be setpurposively and quantitatively exactly by providing and positioning inthe correct location an interlayer with mask material at respectivelysuitable stages of the growth process for forming of III-N templates;

FIGS. 7A and 7B show the change of the curvature of the growth surfacemainly in dependence on the provision and location/positioning of aninterlayer with mask material according to different possibleembodiments of the present invention, in comparison to comparativeexamples; and

FIG. 7C shows the results with respect to curvature of the growthsurface when the templates defined according to FIGS. 7A and 7B aresubjected to a further III-N (GaN) layer growth for producing thickerlayers, in comparison to comparative examples.

Without thus limiting the present invention, the following detaileddescription of the figures, aspects, further embodiments and particularfeatures will clearly illustrate the invention and describe particularembodiments in detail.

DETAILED DESCRIPTION

In the process for producing III-N starting substrates it wassurprisingly found that templates comprising a foreign substrate and atleast one III-N crystal layer, wherein III denotes at least one elementof the third main group of the periodic table of elements, selected fromthe group consisting of Al, Ga and In, can be produced which renderpossible the growth of III-N single crystals having excellentproperties.

In a first embodiment, first, a substrate is provided for the productionof a such template. A suitable substrate may be selected from a groupconsisting of a homo- or hetero-starting substrate, a starting substratehaving a layer of grown III-N crystallites as well as a startingsubstrate having structures formed thereon, for example specific,prepared III-N structures and/or mask structures. According to anembodiment according to the invention shown in FIG. 1, the processaccording to the invention comprises the provision of the substrate bymeans of steps (i) to (iii). Accordingly, first, a starting substrate(i) is provided which is subjected to a desorption step (ii) and anucleation step (iii).

For the provision of a starting substrate a foreign substrate ispreferably suited which for example is made of SiC, silicon, LiAlO₂ orsapphire, particularly preferably is composed of sapphire. Morepreferably a c-plane sapphire substrate is used having a tilt towards(1-100) or (11-20) by 0.1-0.5° and a one-sided epi-ready polishing and apolished and/or preferably lapped backside. A further embodimentprovides for the starting substrate to exhibit a surface structured bylithography or wet chemical or dry chemical etching (e.g. ICP etching).The use of a foreign substrate as starting substrate is of particularadvantage, since the material of the foreign substrate and the grownIII-N material have different thermal expansion coefficients, such thatat change in temperature, a curvature of the substrate, specifically ofthe growth surface, is effected and that this is particularly suitableas starting situation for the process according to the invention.

However, for the present invention, a homo-substrate can also be used asstarting substrate, wherein in this case, a curvature can be introducedfor example by the temperature gradients within the starting material. Afurther possibility is that a homo-starting substrate is used onto whichan—optionally structured—foreign masking layer is subsequently depositedwhich may result in a change of the curvature of the substrate at changein temperature.

A further possibility of providing a suitable starting substrate cancomprise the formation of interlayers or intermediate structures for thepurpose of supporting the later separation from the starting substrate,and/or the formation of a so-called GaN “nano grass” which is based on asubstrate having formed thereon a GaN compliance layer havingnano-column structure, as for example described in WO 2006035212 Al, WO2008096168 A1, WO 2008087452 Al, EP 2136390 A2 and WO 2007107757 A2.

Optionally, the starting substrate may be further pre-treated. Asillustrated in FIG. 1 (ii), a desorption step is preferably carried outwith the provided starting substrate. In this desorption step forexample hydrocarbon residues but also other volatile contaminants can beremoved from the starting substrate or a structured or otherwisepre-treated substrate. During the desorption step the starting substrateis heated in the process to an elevated temperature, preferably to atemperature of 1000 to 1200° C., more preferably to a temperature of1050 to 1150° C., for example approximately about 1090° C. Thereby, dueto the temperature gradient within the substrate, for instance due to adirected heating (typically e.g. by means of a heating from thesubstrate side of the substrate holder or respectively the -deposition,which is opposite to the growing surface) the starting substrate issubjected to a bending, typically with a negative curvature (concave)with respect to the surface onto which subsequently the III-N materialis deposited (cf. FIG. 1). The desorption step may further comprise anitriding with ammonia. A further optional step consists of lowering thetemperature after desorption occurred, for example to a temperaturebetween 400 and 600° C., preferably to a temperature between 450 and550° C. During this cooling the—preferably concave—curvature decreasesagain, for example to the level like in the beginning of the heating tothe desorption step.

The provision and the pre-treatment of a substrate in the process forproducing a template of the present invention may preferably furthercomprise a nucleation step in which crystalline III-N material,especially minute III-N crystallites are grown onto the startingsubstrate. This step is schematically illustrated in FIG. 1 (iii). Thecrystalline III-N material, especially the III-N crystallites serve asseed crystals in the later crystal growth process step. III-Ncrystallites exhibit sizes of e.g. 1 to 40 nm with irregular forms, aregenerally present disorderedly on the starting substrate and suitablyform initially a non-continuous nucleation layer. This nucleation steptypically takes place at temperatures of 400 to 600° C., preferably of450 to 550° C. and more preferably of 500 to 540° C. Since thetemperature for a nucleation is typically lower than that for anoptional preceding desorption, the curvature will decrease from step(ii) to (iii). The state of the provided substrate in step (iii) is thustypically to describe by a relatively low curvature or even (asschematically illustrated in FIG. 1) a missing curvature.

After the provision of a substrate, optionally by the above describedoptional means, such as a nucleation step, that is after the step a)according to the invention, an increase of temperature takes place thento a temperature enabling crystal growth for forming a III-layer.According to the definition of the present invention, the temperature inthis stadium is termed “first” crystal growth temperature of step b)according to the invention; this definition applies independenttherefrom and does not exclude that an increased temperature was appliedbefore, for example in order to carry out pre-treatment, structuring orother processing of the starting material. During the crystal growth ata such first growth temperature, optionally already during the heatingup carried out for increase of temperature, typically a rearrangementprocess of III-N seeds takes place wherein smaller seeds disappear infavour of bigger seeds, gaps form (future vacant lattice positions orvoids at the boundary surface) and the hexagonal habitus of the seedswhich is merely indicated before in the nucleation step is now markedlyincreased. A such step according to the present embodiment isillustrated in FIG. 1 (iv). As schematically indicated in particular instep (iv) of FIG. 1, due to enlargement or coalescence of the III-Nseeds and in the course of the forming layer, there is a curvature ingrowth direction, that is to say in a concave range. The said “first”growth temperature within the reactor is within a temperature range atwhich good growth conditions of the respective III-N materials exist. Inthe case of GaN, this “first” growth temperature is for example withinthe range of 990-1090° C., preferably at 1020-1060° C., and morepreferably at about 1040° C. In the case of AlGaN having an Al-contentof 30% to up to 90%, the “first” growth temperature is for examplewithin the range of 1070-1250° C., preferably at 1090-1130° C., and morepreferably at 1110° C.

At this first growth temperature, growth of crystalline III-N materialonto the substrate takes place. III-N crystallites are growingvertically and laterally. This vertical and lateral growth of the III-Ncrystallites renders possible at sufficient crystallisation time thatcoalescence of the crystallites occurs. During the crystal growth at thefirst crystal growth temperature, the curvature of the substrateincreases. The direction or respectively the algebraic sign of thedistortion/curvature is defined relative to the surface onto which theIII-N material is subsequently deposited. A curvature increasing withthe temperature readily takes place in the case of the use of a foreignsubstrate due to the different thermal extension coefficients of foreignsubstrate and III-N material to be grown. Thereby, the conditions, forinstance by selection of a used foreign substrate relative to the formedIII-N material can be typically selected such that a concave curvatureincreases thereby.

In the case of the use of a homo-substrate, curvature takes place forexample due to temperature gradients within the substrate.

According to the invention, then, for systematic and specificinfluencing of the further curvature development of the template, thegrowth temperature is changed according to step c) according to theinvention in this embodiment. Accordingly, in the case of using sapphireor LiAlO₂ as foreign substrate, the temperature is lowered, and on theother hand, in the case of using SiC or Si the temperature is raised.This takes place preferably at the beginning or during the coalescenceof the growing III-N crystallite. However, in this step the temperatureis maximally changed (i.e. lowered or respectively raised) to an extentthat crystal growth and preferably epitaxial crystal growth can furthertake place. Owing to the change in temperature, the growth process doesnot need to be interrupted. Then, subsequent to the change intemperature on the coalescing III-N crystallites, an epitaxial crystalgrowth takes place in a second temperature range. The change (i.e.lowering or raising) of the temperature sets a temperature difference ΔT(first growth temperature T₁ minus second growth temperature T₂) withinthe reactor, which value is at least 10° C., in particular in the rangeof 10-100° C., preferably at least at 20° C., more preferably within therange of 20-50° C., even more preferably within the range of 25-40° C.and in particular at 30° C.

Accordingly, in the case of the growth of GaN a temperature T₁ ispresent within the reactor which is within the range of 990-1090° C.,preferably of 1020-1060° C., and more preferably at about 1040° C.

Accordingly, the second growth temperature T₂ when using for examplesapphire or LiAlO₂ as foreign substrate is at T₂ lowered compared withthe specifically selected temperature T₁ within the range of 950-1050°C., preferably of 990-1030° C., more preferably at about 1010° C.; onthe other hand, when using SiC or Si as foreign substrate, the secondgrowth temperature T₂ is lowered compared with the specifically selectedtemperature T₁ within the range of 1030-1130° C., preferably of1050-1110° C., more preferably at about 1060° C.

In the case of the growth of AlGaN having an Al-content of up to 30%, atemperature T₁ within the reactor is within the range of 1010-1110° C.,preferably of 1040-1080° C., more preferably of about 1060° C.Accordingly, when using for example sapphire or LiAlO₂ as foreignsubstrate, the second growth temperature T₂ is lowered compared with thespecifically selected temperature T₁ within the range of 970-1070° C.,preferably of 1010-1050° C., more preferably at about 1030° C.; on theother hand, when using SiC or Si as foreign substrate, the second growthtemperature T₂ is lowered compared with the specifically selectedtemperature T₁ within the range of 1050-1150° C., preferably of1060-1100° C., more preferably at about 1080° C.

In the case of the growth of AlGaN having an Al-content of 30% to up to90%, a temperature T₁ within the reactor is within the range of1070-1250° C., preferably of 1090-1130° C., more preferably of about1110° C. Accordingly, the second temperature T₂ when using for examplesapphire or LiAlO₂ as foreign substrate is lowered compared with thespecifically selected temperature T₁ within the range of 1040-1220° C.,preferably of 1060-1100° C., more preferably at about 1080° C.; on theother hand, when using SiC or Si as foreign substrate, the second growthtemperature T₂ is lowered compared with the specifically selectedtemperature T₁ within the range of 1080-1250° C., preferably of1090-1150° C., more preferably at about 1120° C.

While the curvature of the crystalline growth surface which was presentduring growth of crystalline III-N material onto the substrate at thefirst crystal growth temperature is decreased by changing the growthtemperature, it was surprisingly found that when continuing theepitaxial crystal growth within the range of the second growthtemperature—i.e. depending on the used substrate below or above thefirst crystal growth temperature—the curvature of the growth surfacedoes not increase again, but at least keeps constant, preferably furtherdecreases continually or intermittently. In a preferred embodiment, aconcave curvature of the growth surface decreases at changed growthtemperature at continued growth. Thus, in contrast to conventionalprocesses, the curvature decreases despite of growth at the secondgrowth temperature. The second growth temperature may vary within theprescribed, however changed range compared with the first growthtemperature, or it may be kept constant at a specific temperature withinthe changed temperature range.

When the curvature value at the beginning of the crystal growth at thesecond growth temperature is denoted “K_(s)” (K_(start)) and thecurvature value at a later point of time and in particular towards theend of the growth of the III-N layer of the template is denoted “K_(e)”(K_(end)), then the curvature difference (K_(s)−K_(e)) of the templateexhibits a positive algebraic sign. Preferably, K_(s)−K_(e) is at least5 km⁻¹, more preferably at least 10 km⁻¹. On the other hand thecurvature difference (K_(s)−K_(e)) should not be larger than 50 km⁻,more preferably not be larger than 20 km⁻.

By recognizing this behaviour and the correlations involved therewith,by means of the process of the present invention it is possible toproduce a template comprising a first III-N layer which exhibits no oralmost no (substantially no) curvature or is negatively curved. The term“almost no” or respectively “essentially no” curvature is preferablydefined such that the curvature value (K_(e)) at epitaxial growthtemperature is within the range of at most ±30 km⁻.

As can be seen from the above description, according to the inventionthe curvature behaviour can be advantageously influenced and set alreadyin the growth of a single (i.e. one and the same) III-N layer of thetemplate. In particular, the exerting of influence on the curvaturebehaviour is carried out in the denoted steps b) to d) to accordinglygenerate a single III-N layer of the template. Step d) can be carriedout subsequently independently from the described conditions.

Without wishing the present invention to be bound to any theory, it isassumed that in the phase of the coalescence of the first III-N layer,i.e. that short before, at the beginning or shortly after thecoalescence, the density of the seeds on the surface becomes so largethat a closed surface is more favourable for energetic reasons, whereinby means of the expansion of the crystallites taking place in the growthplane, a tensile stress is built up (FIG. 1 (iv)). By means of thedescribed change of the temperature, optionally upon presence of aforeign substrate also by means of the different thermal extensioncoefficients of the starting substrate, the coalescing seeds arecompressed within the plane such that the subsequent III-N crystalgrowth is offered a compressively stressed surface lattice. As a resultof this compressive stress, the growing III-N crystal layer pushes backthe foreign substrate and thereby effects a decrease of the curvature,preferably a decrease of the concave curvature (FIG. 1 (v)). Thisprocess may be continued until achieving a desired minor curvature orthe absence of any curvature or even the generation of a negativecurvature (FIG. 1 (vi)).

By continuing the growth at the second crystal growth temperature,further III-N material is epitaxially grown. The III-component of theIII-N material can now basically be varied if desired, however,preferably attention should be paid to further conditions, in order tonot interfere with the advantageous influencing on the curvaturebehaviour. In particular, the introduction of indium in this phase ofthe process has disadvantages and thus it is preferably refrainedtherefrom, since the growth of Indium-containing materials requiresspecific, particularly low growth temperatures, which result in badcrystal quality, possibly even segregation. In this phase of the growthat the “second” growth temperature, it is thus preferred to dispensewith the addition of In.

Furthermore, in this phase of the growth it is preferred to carry outthe growth within the range of the second growth temperature with III-Nmaterials which, in the case if any aluminum shall be used, exhibit analuminum-content of at most 60%, wherein furthermore, the content (in %of the III-component) within the III-N material of optionally presentaluminum is preferably at most 30%, more preferably at most 20%, or onlywithin the range of 10 to 0%. On the other hand, interestingapplications having an aluminum content of more than 30% are alsoreasonable, for example within the range of 50 to 70%. Indeed, anextremely high aluminum content of more than 70% is basically possible,however, it is rather to be avoided, since such materials have anisolating character and do not suit well for application inopto-electronic components. It is further preferred that theIII-component changes relatively little or rather does not change at allbased on the preceding steps—for instance at the formation of theinitial III-N crystallites onto the substrate as described above—up tothe step of growth within the range of the second growth temperature;for example, it is desirable that in case the III-component is varied—ifany, the change of the III-component is at most 10%. In a specificembodiment, the III-component is not changed within the phase of thegrowth within the range of the second growth temperature, and preferablythe III-N material is GaN.

In the phase of the growth within the range of the second growthtemperature, layer thicknesses of suitably at least 0.1 μm, for examplewithin the range of 0.1-10 μm, preferably of 2-5 μm can be depositedonto the substrate.

In a preferred embodiment of the present invention, all crystal growthsteps described above in the first embodiment, including the optionallyperformed nucleation step, are carried out via organometallic vaporphase epitaxy (MOVPE, Metal-Organic Vapor Phase Epitaxy). Alternativelyor in combination, the crystal growth steps described before can howeveralso be performed via HVPE.

By finishing the crystal growth at the second growth temperature, atemplate is provided according to the invention. The thus obtainedtemplate has advantageous characteristics and features, which will befurther described in the following. As such it is an interestingcommercial object, it can however also be further processed as templatewithin further steps described below, directly subsequently oralternatively indirectly after providing, storing or shipping.

A template for producing further III-N single crystal according to thepresent invention has no or almost no curvature within the temperaturerange of an epitaxial crystal growth. When for substrate sapphire havinga thickness (d_(sapphire)) of 430 μm and for III-N crystal layer of thetemplate GaN of a thickness (d_(GaN)) of 3.5 μm is used or set, then therequirement “essentially no curvature” or “negative curvature” atepitaxial crystal growth temperature means that the template has acurvature K_(T(3.5μm;430μm))<−170 km⁻¹ at room temperature, preferablywithin the range of −170 km⁻¹>K_(T)>−250 km⁻¹ (here, K_(T) means thecurvature of the surface of the template at room temperature), whereinwhen using or setting other layer thicknesses, the curvature value mayvary depending on the respective layer thicknesses analogous to thefollowing simplified Stoney equation, according to which—as long as thefilm (d_(GaN)) is significantly thinner than the substrate(d_(substrate))—the following relationship applies, wherein R=curvatureradius and ε_(xx)=deformation (strain):

1/R=6*(d _(GaN) /d ² _(substrate))*ε_(xx).

Assuming a very thin layer, ε_(xx) is considered to be constant, i.e.when the layer thicknesses change the system reacts with a change of R(the change of ε_(xx) resulting from a change of the curvature isneglected). Thus, the above (partially preferred) ranges applying forthe case K_(T(3.5μm;430μm)) of the curvature value at room temperaturecan be converted as follows in case other values apply for d_(GaN) andd_(substrate):

K _(T(dGaN;dsapphire)) =K _(T(3.5μm;430μm))×(430 μm/d _(sapphire))²×(d_(GaN)/3.5 μm).

For a template according to the present invention this for example meansthat when for 430 m of sapphire and for a 3.5-4 μm thick GaN layer acurvature of −250 km¹ is present, for the same process a curvature of−425 km⁻¹ results for a 330 μm sapphire.

In a further preferred embodiment, the template at room temperatureexhibits a curvature radius within the range of −4 to −6 m for the caseof d_(sapphire)=430 μm and d_(GaN)=3.5 μm.

Another possibility to characteristically describe the product- orstructure-characteristics of the template obtained according to thepresent invention is to specify the strain of the lattice constants orthe stress.

The strain ε_(xx) is defined as follows:

$ɛ_{XX} = \frac{{{lattice}\mspace{14mu} {constant}\mspace{14mu} a} - {{lattice}\mspace{14mu} {constant}\mspace{14mu} a_{0}}}{{lattice}\mspace{14mu} {constant}\mspace{14mu} a_{0}}$

wherein a denotes the actual lattice constant in the crystal and a₀denotes the theoretical ideal lattice constant.

X-ray methods for determining absolute lattice constants are discussedin detail in M. A. Moram and M. E. Vickers, Rep. Prog. Phys. 72, 036502(2009). Thereby the determination is carried out using Bragg's Law

nλ=2d _(h k l) sin θ

initially for the lattice constant c from a 2theta-scan withthree-axes-geometry in symmetrical reflexes such as e.g. 004. The ideallattice constant according to V. Darakchieva, B. Monemar, A. Usui, M.Saenger, M. Schubert, Journal of Crystal Growth 310 (2008) 959-965 isc₀=5.18523±0.00002 Å. The determination of the lattice constant a isthen carried out using the equation,

$\frac{1}{d_{hkl}^{2}} = {{\frac{4}{3}\frac{h^{2} + k^{2} + {hk}}{a^{2}}} + \frac{l^{2}}{c^{2}}}$

also given for example in M. A. Moram and M. E. Vickers, Rep. Prog.Phys. 72 (2009) 036502, from asymmetrical reflexes hkl such as forexample −105 in the 2theta-scan. According to V. Darakchieva, B.Monemar, A. Usui, M. Saenger, M. Schubert, Journal of Crystal Growth 310(2008) 959-965, the ideal lattice constant a₀ for unstressed GaN can beassumed to be a₀=3.18926±0.00004 Å. As to the background of thephenomena of intrinsic and extrinsic stress, among others consideringlattice constants, cf. Hearne et al., Appl. Physics Letters 74, 356-358(2007).

Furthermore, the properties can also be given by the stress σ_(xx),wherein

σ_(xx) =M _(f)·ε_(XX)  (Hooke's formula)

wherein M_(f) denotes the biaxial elastic modulus.

The determination of the stress σ_(xx) is readily possible via Ramanspectroscopy, for example as described in I. Ahmad, M. Holtz, N. N.Faleev, and H. Temkin, J. Appl. Phys. 95, 1692 (2004); therein thebiaxial elastic modulus of 362 GPa is derived from the literature as avalue, wherein a very similar value of 359 GPa can be taken from J.Shen, S. Johnston, S. Shang, T. Anderson, J. Cryst. Growth 6 (2002) 240;thus a suitable and consistent value for the biaxial elastic modulusM_(f) is about 360 GPa.

A template according to the present invention exhibits in thetemperature range of an epitaxial crystal growth a value of ε_(XX)≤0(i.e. including ε_(XX)=0), but in particular of ε_(XX)<0. This value canbe directly determined from an in situ measurement of the curvature.

The template according to the present invention further exhibits at roomtemperature a compressive stress of σ_(xx)<−0.70 GPa. The deformationε_(xx) of the template at room temperature can be set to a value ofε_(xx)≤−0.002 (in particular <−0.002), preferably within the range of−0.002 to −0.004.

A suitable curvature measurement device, which is applicable incombination with an apparatus for vapor phase epitaxy, is for examplethe curvature measurement device of Laytec A G, Seesener Strasse,Berlin, Germany (cf. for example DE102005023302 A1 and EP000002299236A1). These curvature measurement devices are well adapted to be combinedwith available equipments for vapor phase epitaxy, such as MOVPE, HVPEor MBE (Molecular Beam Epitaxy) and furthermore enable a measurement ofthe temperature at the wafer surface.

Accordingly, after the epitaxial crystal growth within the range of thesecond growth temperature, a template is obtained which, based on theabove-described characteristics, is suited to produce crystals ofoutstanding quality and with outstanding features in further epitaxialgrowth steps. The template is thus excellently suited for the furtheruse, it may also as such be provided, stored or shipped for further use,or it may be directly further used in an entire process.

Thus, in a further embodiment of the present invention, III-N singlecrystals can be produced which are obtained by carrying out subsequentto the crystal growth step at the second growth temperature—without orwith interruption in between—an additional epitaxial crystal growth onthe template obtained according to the invention for forming of III-Ncrystal at a crystal growth temperature which can be selectedindependently from the said first and second crystal growth temperatures(corresponding to step e) according to the invention of thisembodiment). Thereby, the crystal growth temperature in step e) can befreely selected depending on III-N material desired for the epitaxiallayer to be formed, and thus can be within the range at which there aregood growth conditions of the respective III-N material.

Now, in step e) according to the invention, other conditions of thecrystal growth can also be freely selected. For example, III-N materialscan be grown which III-component can be freely selected. Furthermore,now indium may also be contained as III-component. Moreover, materialsmay be used which Al content is more than 60%.

Accordingly, in step e) according to the invention of this embodiment,at least one (optionally more) GaN-, AlN-, AlGaN-, InN-, InGaN-, AlInN-or AlInGaN-layer(s) can be deposited for producing accordingly thickerIII-N layers or III-N single crystals. Preferably, the III-N crystallayer onto the substrate as well as the III-N crystal epitaxially grownthereon form a purely binary system, e.g. GaN, AlN or InN, or the III-Ncrystal layer onto the substrate is a purely binary system, inparticular GaN, and the III-N crystal epitaxially grown thereon is abinary or ternary III-N material which can be freely chosen, inparticular again binary GaN.

Step e) may directly follow step d), alternatively, the process may beinterrupted therebetween. Furthermore, it is possible to carry out stepsd) and e) within the same reactor, alternatively to change the reactorbetween said steps. This renders possible to grow the III-N singlecrystal by a growing method different from that used in the productionof the provided template, in order to select optimum conditions for therespective steps. For instance, the additional epitaxial crystal growthon the template produced according to the invention is preferablycarried out via HVPE. The advantageous selection of the step e) underHVPE conditions renders possible high growth rates and accordingly theobtaining of thicker layers. However, also all steps of the processrelating to the entire growth of the III-N layer can be carried out in asingle equipment using a particular growth technique, for example onlyvia HVPE.

A further aspect of the present invention is a process for producingIII-N single crystals, wherein III denotes at least one element of thethird main group of the periodic table of the elements, selected fromAl, Ga and In, wherein the process comprises the following steps:

-   aa) providing a template comprising a starting substrate and at    least one III-N crystal layer, wherein the starting substrate and    the at least one III-N crystal layer are formed such that the    template within the temperature range of an epitaxial crystal growth    is not or almost not curved or is negatively curved, and-   bb) carrying out an epitaxial crystal growth of III-N crystal at a    crystal growth temperature at which the template is not or    essentially not curved or is negatively curved.

For the reasons already described in the preceding embodiments, it isalso preferred here to add no In in steps aa) and bb). If desired,additional epitaxial crystal growth can optionally follow for forming ofIII-N crystal at a crystal growth temperature which can be independentlyselected from the said crystal growth temperatures of the step bb),wherein in this step of continued growth, Indium may optionally beadded. Subsequently, optionally a separating of the III-N single crystallayer(s) from the substrate is possible.

This aspect of the invention starts from the alternative solutionprinciple of minimizing or eliminating altogether the risk of crackformation by the preconditions specified in the steps aa) and bb).

For providing a template for step aa) it may for example be referred tothe above description concerning the formation of the template accordingto the invention.

For example as possibilities for the provision of a template for stepaa), applicable as alternatives or optionally in combination, theabove-described method of the temperature change during the growth orrespectively the method of using a mask material interlayer in theforming of the template-III-N layer, likewise described above, can beapplied. While for the former method it can be referred to the detaileddescription above, for the latter method exemplary embodiments areschematically shown in FIG. 6.

In the FIGS. 6A and 6B in the same step (1) initially the provision ofthe respective substrates 100A and 100B is shown. The respectivesubstrates can optionally be pre-treated as described above, inparticular said substrates can respectively be subjected to a desorptionstep and a nucleation step. In a nucleation step crystalline III-Nmaterial 101A or respectively 101B is formed, in particular minute III-Ncrystallites on the starting substrate (cf. step (2) of FIGS. 6A and6B), which serves as seed crystals in the later further III-N crystalgrowth. The further steps can vary as concerns time point andposition/location of the layer of the mask material and the resultingconsequences thereof, as is illustrated respectively separately in FIGS.6A and 6B. In the embodiment shown in FIG. 6A an interlayer made of maskmaterial 102A is directly deposited already on the nucleation layer101A, still before coalescence of the crystallites starts. In a furthermodification (not specifically shown here) this deposition of theinterlayer is carried out not directly on the nucleation layer but onlyafter a very short phase of a III-N growth, but still very close in thenanometer range to the nucleation layer, for example, at a distance in arange of up to 30 nm. In this distance range selected very close to thenucleation layer the subsequent steps occur practically analogous to theform shown in FIG. 6A. In the embodiment shown in FIG. 6B, on thenucleation layer 100B initially a III-N growth is carried out for aparticular, generally still relatively short time, for example until asmall thickness of the crystalline III-N layer 103B of 30 nm or beyondand suitably up to 300 nm or below has formed, preferably up toapproximately 100 nm, more preferably up to approximately 50 nm, andonly then an interlayer made of mask material 102B is deposited in thecorresponding distance from the nucleation layer of the substrate.Suitably and advantageously the deposition of the denoted interlayer102A or respectively 102B is carried out in the same reactor with aprocess which is compatible with the technique for growing the III-Nlayer of the template.

For example, a silane gas and ammonia is flown into the reactor andreacted together at a suitable pressure and a suitable temperature offor example 800° C. to 1200° C., preferably at about 1050 to 1150° C.and is deposited in the form of Si₃N₄ and optionally furtherstoichiometric or over- or substoichiometric Si_(x)N_(y) compositions onthe prepared substrate (100A; 101A). The step of depositing maskmaterials other than SiN, such as for example TiN, Al₂O₃, SiO₂, WSi, andWSiN, can readily and accordingly be adjusted. The thus formed masklayer 102A or respectively 102B can exhibit different forms. It isgenerally very thin, suitably in the nanometer or sub-nanometer range,for example below 5 nm, more preferably below 1 nm, in particular downto below one monolayer (i.e. 0.2 to 0.3 nm or less), and it can behomogeneously distributed on the surface and may form a continuouslayer, alternatively however it exhibits rathermicroscopic/nano-structured gaps or a discontinuous structure (shown inthe drawing schematically in the form of a dashed layer 102A orrespectively 102B). After depositing the interlayer with mask material,the (continued) growth of a III-N layer 104A, 104B (stage (4) in FIG.6A/6B) is carried out immediately thereafter until the template at theend of the growth (stage (5) in FIG. 6A/6B) exhibits a III-N layer 105A,105B with desired thickness in the range from 0.1 to 10 μm, preferablyin the range from 3 to 10 μm. According to the invention it is made surealso under this embodiment that the characteristics curvature (measuredat the growth surface) and/or stress of the III-N layer of the templateare favourably influenced and advantageously used for subsequentprocesses.

According to the invention it is effected that the curvature of thetemplate decreases during the subsequent further growth of a growingIII-N layer 104A or respectively 104B, as shown schematically in therespective steps (4) of FIGS. 6A/6B. Different from a situation withoutdeposition, according to the invention by means of the mask layer102A/102B at suitable location/position even a decrease of the curvatureis achieved and thus a curvature difference K_(s)−K_(e)≥0 is observed.

By the imprinting of lattice deformation and of compressive stressaccording to the invention, as a result the condition of the templateprovided in the step aa) can alternatively be defined in that itexhibits a value of ε_(xx)≤0 (i.e. including ε_(xx)=0) at growthtemperature, but in particular a value of ε_(xx)<0, wherein the valuelies preferably within the range of 0>ε_(xx)>−0.0005. Accordingly, atroom temperature, a compressive stress of σ_(xx)<−0.70 GPa is present,and thus, the strain ε_(xx) at room temperature of the template exhibitsa value of ε_(xx)<0, preferably in the range 0>ε_(xx)≥−0.003, morepreferably in the range of <−0.002, in particular in the range of −0.002to −0.004 and better still in the range of −0.0020≥ε_(xx)≥−0.0025.

The epitaxial crystal growth of the III-N crystal according to step bb)of this embodiment may be carried out according to step d) of the abovedescribed embodiment; in this connection, it is explicitly referred tothe description corresponding thereto. In a preferred embodiment, thisgrowth is carried out via HVPE. In particular, again, the III-N materialcan be freely selected. However, it is preferred that the III-N crystallayer onto the substrate as well as the III-N crystal layer epitaxiallygrown thereon, which now forms the III-N crystal, are composed of thesame material, or that the change of the III-component is less that 10%.Furthermore, it is possible that no material exchange is carried out forthe III-component in the III-N crystal layer onto the substrate as wellas the III-N crystal epitaxially grown thereon. If no foreign substrateis used as starting substrate but a homo-substrate, then a furtherpossible embodiment arises wherein the starting substrate, the III-Ncrystal layer onto the substrate as well as the III-N crystalepitaxially grown thereon are composed of the same III-N material.

According to the invention in the process for producing III-N singlecrystals according to the embodiments described above an epitaxialcrystal growth on the provided template can be carried out such thatafter finishing the epitaxial growth (step e) or respectively bb) of thedescribed embodiments), III-N single crystals having layer-thicknessesof at least 1 mm, preferably of at least 5 mm, more preferably of atleast 7 mm and most preferably of at least 1 cm are obtained.

After completion of the epitaxial crystal growth for producing a III-Nsingle crystal, the III-N single crystal can optionally be separatedfrom the substrate (optional step f) or respectively cc)). In apreferred embodiment, this takes place via self-separation, such asduring the cooling from a crystal growth temperature. In a furtherembodiment, the separation of III-N single crystal and the substrate canbe performed by grinding-off, sawing-off or a lift-off process.

When the epitaxially grown III-N single crystal exhibits a sufficientlylarge thickness, wherein a so-called III-N boule or ingot is obtained,it is possible to separate this single crystal for forming a multitudeof individual thin disks (wafers) by using suitable methods. Theseparation of the single crystals comprises common methods for cuttingor sawing of III-N single crystals. The wafers thus obtained areexcellently suited as a basis for producing semiconductor devices andcomponents, for example opto-electronic and electronic components. Thewafers produced according to the present invention are well suited foruse as power components, high-frequency components, light-emittingdiodes and in lasers.

A further aspect of the present invention is the provision of templatesor respectively III-N single crystals adhering to foreign substrate.These products are available via the processes described above and arein particular suitable as basis material for producing thicker III-Nlayers or respectively boules (bulk crystals) and as basis for componentproduction. These products provided according to the invention have theabove described parameters with regard to the III-N single crystal,among others the given values ε_(xx)≤0 (i.e. including ε_(xx)=0), inparticular a ε_(xx)-value of <0, preferably within the range of0>ε_(xx)>−0.0005 in the temperature range of an epitaxial crystal growthor respectively at room temperature.

In all of the process stages, in particular for the actual, epitaxiallygrown III-N layers and correspondingly in the III-N single crystalaccording to the present invention, the inclusion of dopants ispossible. Suitable dopants comprise n- as well as p-dopants and maycomprise elements selected from the group consisting of Be, Mg, Si, Ge,Sn, Pb, Se and Te. For semi-isolating material suitable dopants cancomprise elements selected from the group consisting of C, Fe, Mn andCr.

In a further preferred embodiment, the obtained III-N single crystal iscomposed of gallium nitride, and this crystal exhibits in growthdirection a lattice constant a within the range of <a₀, in particularwithin the range of 0.31829 nm<a≤0.318926 nm. As reference value of thelattice constant a₀ of GaN, here the value of a₀=0.318926 nm can beassumed (cf. V. Darakchieva, B. Monemar, A. Usui, M. Saenger, M.Schubert, Journal of Crystal Growth 310 (2008) 959-965. This correspondsapproximately to a lattice constant c within the range of0≤ε_(zz)<+0.001.

EXAMPLES Example 1

Example 1 relates to GaN growth carried out on a sapphire startingsubstrate. The individual steps of the process are illustrated in FIGS.2 and 3. FIG. 2 shows the change of the curvature of the growth surface(right ordinate; lower line) depending on process steps 1 to 6(corresponding to steps (i) to (vi) of the FIG. 1) and respectivelyapplied temperature (left ordinate; upper and middle line).

In FIG. 3 the following parameters are plotted versus time: the changeof the reflexion at 950 nm (top left diagram) as well as the temperature(top right diagram, wherein the upper curve expresses the set orrespectively controlled process temperature and the lower curveexpresses the real temperature at the wafer location), and the change ofthe curvature of the growth surface (lower diagram). The individualprocess steps are indicated between the two partial diagrams of FIG. 3,wherein in the following, the process step from FIG. 3 denoted with theterm “GaN-layer” is termed “crystal growth.”

The process steps indicated in FIG. 3 have the following correspondencesof the FIG. 1:

“Desorption” corresponds to step (ii) of FIG. 1;“GaN-nucleation” corresponds to step (iii) of FIG. 1;“recrystallisation” corresponds to step (iv) of FIG. 1;“T-ramp” corresponds to step (v) of FIG. 1;“GaN-layer” corresponds to step (vi) of FIG. 1.

The measurement of the curvature of the growth surface is carried out insitu. The measurements were carried out with an EpicurveTT curvaturemeasurement device by the company LayTec (Berlin, Germany) which allowsto simultaneously obtain data on temperature, reflection and curvatureof the growth surface.

In the following, details of a first embodiment for producing a templateaccording to the invention are described. As growth technique a MOVPE isused. The temperatures given here relate to the temperature of theheaters; the temperature at the template or respectively the crystal islower for about 200 K (cf. FIG. 2: here the nominal heater temperatureis denoted by the upper line and the measured temperature of the wafersupport is illustrated by the middle line).

Foreign Substrate:

c-plane sapphire substrate430 μm thicknessunstructured

Desorption Step in MOVPE Multi Wafer Reactor

Reactor: Aix2600 G3 or G4

Reactor pressure: 200 hPaHeating: from 400° C. to 1180° C. in 13 minReactor temperature: 1180° C.Process temperature duration: 10 min in H₂ atmosphere

Cooling to 540° C.

Nucleation Step

Gas flows: 25 sccm TMGa, 1 slm NH₃

Cooling to 530° C.

Reduction of the satellite rotation by reducing the flow to 100 sccmrespectively Opening of the valves

Nucleation: 8 min 30 s

Increase of the ammonia flow to 5.5 slm

Recrystallisation

Heating from 530° C. to 1240° C. in 8 minSimultaneous ramping of the reactor pressure to 600 hPaCeiling temperature: 370° C. andGas flow: 100 sccm TMGa, 16 slm NH₃. Thereafter, setting of theconditions for coalescence:Reactor pressure: reducing from 600 hPa to 500 hPa in 10 minGas flow for increasing the deposition rate: 200 sccm TMGCeiling temperature: 310° C.

T-Ramp and Crystal Growth

Cooling from 1240° C. to 1210° C. in 10 minReactor pressure: 500 hPa, H₂-atmosphereGas flows: 200 sccm TMGa, 5500 sccm NH₃Crystal growth time: 90 min

End of Growth and Cooling

Switching of heating and TMGa flowReducing NH₃: 5500 sccm to 4 slm in 5 minSwitching-off: NH₃-flow under 700° C.Switching-over: NH₃-flow to N₂-flow

As can be gathered from FIGS. 2 and 3, by applying the temperature ramp,i.e. reducing from the “first” to the “second” growth temperature, the(here: concave) curvature of the substrate (having GaN materialinitially formed thereon) is reduced to a curvature value K_(s), andthis (here: concave) curvature K_(s) is further reduced at the “second”growth temperature in the course of the GaN crystal growth up to acurvature value K_(e) at the end of the GaN crystal growth. Thefollowing relationship applies: curvature of the sapphire K_(s)(pos. 5in FIG. 2; also shown by arrow and point)>curvature of the GaN/sapphireK_(e)(pos. 6 in FIG. 2; also shown by arrow and point), i.e. the GaNlayer is intrinsically, compressively stressed. Here, the differenceK_(s)−K_(e) is ≥5 km⁻¹, however, it may still be set higher, e.g. ≥10,≥15 or ≥20 km⁻¹, by means of setting respectively higher temperaturedifferences.

Curvature K_(s): 36 km⁻¹Curvature K_(e): 16 km⁻¹Values of the obtained GaN crystal (after cooling):3.5 μm thick GaN layer (after 90 min MOVPE)Lattice constant a: 0.31828ε_(xx) value: −0.00203σ_(xx) value: −0.73 GPaHalf-life width of the rocking curve: 200 arcsec at 002 reflex and 380arcsec at 302 reflex at an entry slit of 0.2 mm×2 mm of a Philips X'PertPro.

At the end at growth temperature in the “second” growth temperature, theGaN/sapphire template thus has a curvature value K_(e) at crystal growthtemperature of only at most 30 km⁻¹. Thus, a state of the template isprovided whereupon this has no or almost no curvature within thetemperature range of an epitaxial crystal growth.

The further procedure may be selected as required. For example, anadditional epitaxial crystal growth may follow for forming of thickerIII-N crystal. A such continued growth can be carried out at a crystalgrowth temperature which can in principle be selected independent fromthe mentioned first and second crystal growth temperatures.

Alternatively, the template may be cooled to room temperature and beplaced at a later point of time, optionally at another place or inanother reactor, in order to be used again or respectively processedagain as required at a later stage. As derivable from FIGS. 2 and 3, inthe present Example, the curvature tilts in the course of cooling toroom temperature from concave to convex. By a cooling, an additionalextrinsic compressive stress is added to the above described,intrinsically, compressive stress of the GaN layer. Thereby, comparedwith the Comparative Example described below, the compressively stressedIII-N layer obtained according to the invention is strongercompressively stressed than a III-N layer stressed only by extrinsiccompression.

As has surprisingly turned out, this compressively stressed III-N layerobtained according to the invention has a positive effect on thecurvature behavior at room temperature and in particular on the optionalre-heated state.

Because in case the such obtained template is heated again to operatingtemperature, i.e. is heated to a suitable epitaxial growth temperature,the state of a low or missing curvature (K_(e)) is recovered, such thatthe advantageous state for further epitaxial III-N crystal growth isattained again.

Example 2

This example relates to GaN growth onto a foreign substrate which doesnot, like in Example 1, have a higher (sapphire) but only a thermalexpansion coefficient lower than GaN, for example Si or SiC. Here, theperformance can be effected according to Example 1, however with thedifference that the indicated relevant change in temperature (“T-ramp”)does not go down but goes up, as schematically illustrated by acorresponding FIG. 4. Though, the intended effect is qualitatively thesame, namely a decrease of the curvature K_(s)−K_(e)>0 in the course ofthe further III-N growth of the template. Once the growth of thetemplate is finished, either direct or indirect (without or withinterruption) further growth onto this template may subsequently follow.Optionally, the template is cooled to room temperature, and in this casethe template additionally curves/stresses thermically (extrinsically) atsuch a cooling to room temperature by nature, now further in the concavedirection, as shown in FIG. 4. For the further use or respectivelyprocessing according to the invention it is however important that theadditional thermic, extrinsic stress/curvature is reversible, andtherefore, in case the template according to the invention is heatedagain to working temperature for epitaxial growth, a state of a low ormissing curvature (K_(e)) is recovered (cf. FIG. 4, state of thecurvature before “cooling”), such that the most advantageous state forfurther epitaxial III-N crystal growth is attained again.

Comparative Example

In a Comparative Example, the same procedural conditions may be applied,except that the temperature is not decreased.

FIG. 5 shows typical in situ data of the MOVPE growth of GaN on sapphirein such a Comparative Example, i.e. without application of awell-targeted temperature ramp. Analogous to FIGS. 3 and 4, thefollowing parameters are plotted versus time: the change of thereflexion at 950 nm (top left diagram ordinate) as well as thetemperature (top right diagram ordinate, wherein the upper curveexpresses the set or respectively controlled process temperature and thelower curve expresses the real temperature at the wafer location), andthe change of the curvature of the growth surface (lower diagram).

The lower picture shows the development of the curvature during theprocess for five different sapphire substrates. The arrows indicate thecurvature values K_(s) and K_(e) to be referred to (cf. also thepositions of the temperature curve indicated by quadratic points), andK_(s)−K_(e)<0 does apply, i.e. the GAN layer is intrinsically stressedin a tensile manner. By means of a cooling, this stress of the GaN layeris superimposed by an extrinsically compressive stress.

Typical values for the Comparative Example:

Curvature K_(s): 50 km⁻¹,Curvature K_(e): 70 km⁻¹,

ε_(XX)-value: −0.0015, σ_(XX)-value: −0.55 GPa. Example 3

Onto the template produced according to Example 1 and in parallelthereto onto the template produced according to the Comparative Example,in a HVPE plant under conventional process parameters, about 1 mm thickGaN boules are grown respectively. Thereby, compared with theComparative Example, the marginal compressive stress set in the templateaccording to Example 1 has a positive effect in that the tendencytowards crack formation is lower.

Example 4

In the following Examples 4 and 5 further embodiments are described withwhich alternatively a template can be provided which in the temperaturerange of an epitaxial crystal growth is not or is essentially not curvedor is negatively curved, which template is then suited very well forcarrying out a further epitaxial crystal growth of III-N crystal. Asgrowth technique for example a MOVPE on pre-treated sapphire (which issubjected to a desorption and a nucleation) is used with the detailsgiven in the following. The temperatures given here relate to thenominally set temperature of the heaters; the temperature at thetemplate or respectively the crystal is lower, in some cases up to aboutapproximately 70 K lower.

Reactor:

MOVPE reactor Aixtron 200/4 RF-S, single wafer, horizontal

Foreign Substrate:

c-plane sapphire substrate, off-cut 0.2° in m-direction430 μm thicknessunstructured

Desorption Step (FIG. 6 (1); 100)

Reactor pressure: 100 mbarHeating: from 400° C. to 1200° C. in 7 minReactor temperature: 1200° C.Process temperature duration: 10 min in H₂ atmosphere

Cooling to 960° C.

Nucleation Step (FIG. 6 (2); 101)

Gas flows: 25 sccm trimethyl aluminium (TMAl), bubbler: 5° C., 250 sccmNH₃

Cooling to 960° C.

Opening of the valves

Nucleation: 10 min

Increase of the ammonia flow to 1.6 slm

T-Ramp; Optionally Crystal Growth (FIG. 6 (2) to Before (3); 103)

Heating from 960° C. to 1100° C. in 40 secReactor pressure: 150 mbar, H₂ atmosphereGas flows: optionally 16-26 sccm trimethyl gallium (TMGa), 2475 sccm NH₃Crystal growth time: 0-10 min (corresponding to 0-300 nm)

SiN Deposition (FIG. 6 (3); 102)

Gas flows: 0.113 μmol/min silane, 1475 sccm NH₃

No TMGa

Pressure: 150 mbar

Temperature: 1100° C. Duration: 3 min

Further Crystal Growth: (FIG. 6 (4); 104)

1100° C.

reactor pressure: 150 mbar, H₂ atmospheregas flows: 26 sccm TMGa, 2000 sccm NH₃crystal growth time 90-240 min, corresponding to 3-10 μm GaN thickness

Growth End and Cooling: (FIG. 6 (5)-(6))

Switching-off of heating and TMGa flowLowering of NH₃: 2000 sccm to 500 sccm in 40 secSwitching-off: NH₃ flow under 700° C.Switching-over: NH₃ flow to N₂ flow

FIG. 7A shows the course of the curvature at growth temperature (1350°K), plotted against the thickness of the grown GaN layer and thus in thetemporal course, distinguished respectively according to distance of theSiN (Si_(x)N_(y)) with respect to the AlN nucleation layer. In thisrespect the zero point relates to the beginning of the continued growthof the III-N layer 104A, 104B (i.e. after stage (3) and before orrespectively during stage (4) in FIGS. 6A/6B). The curvature behaviourcan be purposively and precisely controlled. The following Table 1 givesthe in situ, i.e. measured at growth temperature, ε_(xx) values and thecurvature values C (km⁻¹) measured at room temperature and the ε_(xx)values at room temperature determined from C towards the end of thetemplate production with respective thicknesses of about approximately 7μm.

TABLE 1 distance AlN thickness C @ RT and SiN (μm) ε in-situ (km⁻¹) ε @RT  0 nm 7.21 −6.00E−04 −396 −2.27E−03 15 nm 7.09 −4.50E−04 −365−2.13E−03 30 nm 6.76 −4.00E−04 −367 −2.24E−03 60 nm 6.73  1.10E−04 −298−1.83E−03 90 nm 6.81  1.00E−04 −299 −1.82E−03 300 nm  7.29  2.50E−04−293 −1.66E−03

Example 5 and Comparative Examples

On selected templates produced according to Example 4 for which GaNlayers with SiN interlayers directly on the nucleation layer (sample A)or after a very small (15-30 nm; sample D) or larger (300 nm; sample C)distances were deposited or according to Comparative Examples for whichGaN was grown without SiN (sample B) or on low temperature GaNnucleation layer (sample E), the curvature was followed analogous toExample 4, namely in the range of a MOVPE growth to approximately 7 μmas shown in FIG. 7B, or during performing further HVPE growth toapproximately 25 μm as shown in FIG. 7C. The results of the FIGS. 7B and7C show once more the significantly better results regarding setting andbehaviour of the curvature of the templates according to the invention(A), (C) and (D) compared to the comparative templates (B) and (E)without SiN interlayer.

What is claimed is:
 1. A III-N single crystal adhering to a substrate,wherein III denotes at least one element of the third main group of theperiodic table of the elements, selected from the group of Al, Ga andIn, wherein the III-N single crystal exhibits, within a temperaturerange of an epitaxial crystal growth, a value (i) of deformation ε_(XX)in the range of <0.
 2. A III-N single crystal adhering to a substrate,wherein III denotes at least one element of the third main group of theperiodic table of the elements, selected from the group of Al, Ga andIn, wherein the III-N single crystal exhibits at room temperature avalue (ii) of deformation ε_(XX) in the range of <0.
 3. The III-N singlecrystal according to claim 2, in the form of a III-N single crystallayer deposited on the substrate to form a template, the III-N singlecrystal layer having a thickness of 0.1-10 μm.
 4. The III-N singlecrystal according to claim 2, wherein the value (ii) of deformationε_(XX) is in the range of ≤−0.002.
 5. The III-N single crystal accordingto claim 2, wherein the value (ii) of deformation ε_(XX) is in the rangeof between −0.002 and −0.004.
 6. The III-N single crystal according toclaim 2 wherein the thickness of the III-N single crystal layer iswithin the range of 2 μm to 5 μm.
 7. The III-N single crystal accordingto claim 2, wherein the thickness of the III-N single crystal layer isat least 1 mm.
 8. The III-N single crystal according to claim 7 whereinthe value (ii) of deformation ε_(XX) is between 0 and −0.0005.
 9. TheIII-N single crystal according to claim 2, wherein the III-N singlecrystal exhibits, within a temperature range of an epitaxial crystalgrowth, a value (i) of deformation ε_(XX) in the range of <0.
 10. TheIII-N single crystal according to claim 1, wherein ε_(XX) is determinedwith an X-ray measurement of the absolute lattice constant.
 11. TheIII-N single crystal according to claim 1, wherein the substrate isselected from the group consisting of LiAlO₂, sapphire, SiC and Si. 12.The III-N single crystal according to claim 2, wherein ε_(XX) isdetermined with an X-ray measurement of the absolute lattice constant.11. The III-N single crystal according to claim 2, wherein the substrateis selected from the group consisting of LiAlO₂, sapphire, SiC and Si.